Agricultural crop and field sprayers, both pull-type sprayers and self-propelled sprayers, need to apply a correct and constant amount of liquid per hectare rate at any moment in time. Varying speed of the sprayer, varying wind conditions, overlapping areas, deactivation and activation before and after headland, and defect sensors pose challenges with respect application rate, acceptable pressure range and flow stability. Further, too high pressures result in a too fine spray that blows away with the wind and too low pressure causes insufficient spread and atomizing, thus there is a need to provide constant boom pressure and droplet size throughout the speed range. Ideally, the operator can drive according to field conditions without concern for pressure deviation, so average field speed can increase.
In known sprayers, the flow and pressure is controlled with a motorized regulation valve. This regulation valve typically has a movable valve member that defines a controllable flow opening and an electric drive motor coupled to the valve member via a reduction gear. The opening area of regulation valve can be varied continuously between two extreme positions through operation of the electric drive motor.
This type of regulation valve is precise, robust and reliable but is, in contrast to solenoid valves and the like slow to change position. This type of regulation valve can handle large flow with a relatively small pressure drop.
Solenoid valves without servo amplification are difficult to use in an agricultural crop and field sprayer because the valve is large and the required strength of solenoid is not practical. Servo amplification is normally not used since the sprayer fluid has, due to the chemicals/substances added to the water, properties that do not harmonize with hydraulic servo systems due to e.g. deposits in the servo system. Solenoid valves without servo are not suited to handle large flow with a relatively small pressure drop.
The slow response to demand of the regulation valve renders it difficult for the sprayer control system to respond adequately to changes in the operating conditions of the sprayer. The fact that this known type of sprayer typically operates with a fixed displacement pump increases the challenges for the control system of the sprayer.
US2009112372 discloses a spray control system for controlling an agricultural sprayer that includes a controller, a plurality of sensors and feedback means, and an output means for controlling the application system of the sprayer. The controller receives inputs from the operator through a user interface, and/or various feedback signals from the sensors of the system (e.g., a flow meter, or a pressure transducer). After processing these inputs, the controller sends signals to other components of the sprayer, such as, the pump, the storage means, the boom sections, and/or the nozzles, to maintain or change their operating conditions. This sprayer provides a spray control system that allows selection between a flow volume-based closed loop feedback control system, and pressure-based closed loop feedback control system, i.e. a system with two feedback source. The controller overrides the choice of the operator in the selection of the feedback control method, where the selected feedback source has failed (e.g., due to a component failure).